In the United States, arsenic concentrations in ground water are sometimes too high for potable drinking water, particularly in the West and parts of the Midwest and Northeast. Data collected by the U.S. Geological Survey indicate at least 10 percent of wells had arsenic concentrations in excess of 10 ug/l, which may also be expressed as micrograms per liter or parts per billion (ppb.) Low levels of arsenic exposure can cause a number of adverse health effects, including decreased production of red and white blood cells, abnormal heart function, blood vessel damage, liver or kidney injury, and impaired nerve function. It has also been linked to skin cancer, bladder cancer, and lung cancer.
The major source of the arsenic contamination is from naturally-occurring subsurface minerals. In its geologic occurrence, arsenic is typically found at high levels in primary sulfide minerals, such as arsenopyrite, pyrite, pyrrhotite, orpiment, and realgar. It is also found in trace amounts in silicate minerals. Arsenic is typically found in groundwater as arsenite (AsO33−) (in which arsenic is in the trivalent state) or arsenate (AsO43−) (in which arsenic is in the pentavalent state) ions derived from the leaching of naturally occurring subsurface materials and to a lesser extent from anthropogenic contamination.
Based on a review of the new data, the National Research Council (NRC) committee recommended that the Environmental Protection Agency (EPA) lower its standards on the amount of arsenic allowed in drinking water as soon as possible. The committee found a combined cancer risk of one in 100 from drinking water with the currently allowed maximum level of arsenic. In mid-January, 2001, the EPA announced a tougher drinking water standard for arsenic. The new rule lowers the standard from 50 ppb to 10 ppb and is expected to affect 10 percent of the nation's community drinking water systems.
There are various techniques for removing arsenic and other contaminants from aqueous streams. These methods include ion exchange, adsorption using activated alumina, or flocculation using iron hydroxides.
Ion exchange has long been known to remove arsenic from drinking water. In the process, anionic species, such as arsenic, are exchanged onto a polystyrene resin at reactive sites. Strong-base anionic exchange resins will remove pentavalent arsenic but are generally not effective for the removal of trivalent arsenic because trivalent arsenic is normally present as nonionic arsenous acid. Sulfate ions may occupy exchange sites on the resin in preference to arsenic ions, which is particularly a problem for ground waters high in total dissolved solids (TDS). The regeneration of the ion exchange media will also likely generate a hazardous waste due to the high arsenic concentration.
Activated alumina is especially useful for treating water with very high TDS. However, the effectiveness of activated alumina is highly pH sensitive. The adsorption process involves surface complexation and exchange of hydroxide ions for contaminants. Like ion exchange, activated alumina absorbs only ionized forms of arsenic and thus any trivalent arsenic must be oxidized to pentavalent arsenic to be removed from water. The solution pH is more critical to arsenic removal by activated alumina adsorption than it is for other water treatment processes such as ion exchange and iron-hydroxide coagulation. At high pH, silicic acid dissociates to produce H3SiO4−, which competes with arsenic species for sorption sites. Depending on feedwater pH, groundwater may require substantial amounts of acid to adjust the feedwater pH to below-neutral conditions for optimal efficiency.
The coagulation/filtration process relies on the coprecipitation or sorption of pentavalent arsenic onto freshly precipitated Fe(OH)3 solids. Removal of arsenic by iron hydroxide precipitation depends on pH, with more arsenic being removed at a lower pH. Silica may interfere with arsenic removal by adsorbing on iron hydroxide at a high pH. Disposal of the arsenic-containing coagulation sludge may be a concern if the arsenic is leachable from the residuals and does not pass the Toxicity Characteristic Leaching Procedure (TCLP).